This invention relates to optical communication systems, and more particularly, to systems employing multimode optical waveguides for transmitting optical signals with low dispersion.
High capacity communication systems operating around 10.sup.15 Hz are needed to accommodate future increases in communication traffic. These systems are referred to as optical communication systems since 10.sup.15 Hz is within the frequency spectrum of light. Optical waveguides, which are the most promising medium for transmission at such frequencies, normally consist of an optical fiber having a transparent core surrounded by transparent cladding material having a refractive index which is lower than that of the core.
The propagation of light waves in optical waveguides is governed by laws of physics similar to those that govern microwave propagation and therefore can be studied in terms of modes, each of which has its own propagation and distribution characteristics. Single mode waveguides are advantageous in that they are capable of propagating optical signals with very low dispersion, but due to the low numerical aperture and small core size of such fibers, lasers must be employed to inject optical signals into these waveguides.
Multimode waveguides have larger core diameters and larger numerical apertures than single mode waveguides and are therefore often the preferred medium for the transmission of optical signals since they can accept light from incoherent, broad spectral width sources such as light emitting diodes. However, thousands of modes propagate in multimode waveguides, each mode traveling at a slightly different group velocity. A short input pulse that is shared by many guided modes thus splits up into a sequence of pulses that arrive at the output end of the waveguide at different times. This type of pulse dispersion is the dominant cause of dispersion in typical multimode waveguides. Pulse dispersion from wavelength dependent effects, viz. material dispersion and dispersion within each mode due to the wavelength dependence of the modal group velocity, are usually present to a lesser extent than the first mentioned cause of dispersion resulting from modal velocity differences.
As a result of mode coupling or the use of graded index profiles, or some combination of these two effects, pulse dispersion resulting from group velocity differences among modes can be reduced significantly so that the wavelength dependent effects become the dominant source of dispersion if sources with broad spectral widths, i.e., sources having widths greater than about 20A, are used,
Two different effects have previously been employed to reduce dispersion resulting from group velocity differences among modes. U.S. Pat. Nos. 3,666,348 and 3,687,514 teach methods of reducing dispersion in multi-mode waveguides by deliberately increasing mode conversion opportunities along the wave path such that the energy is forced to propagate in different modal configurations. The energy thus tends to arrive at the output more nearly at the same average time. In these two patents mode coupling is caused by introducing changes in such fiber parameters as core radius and/or changes in the direction of the fiber axis. Mode coupling can also be caused by external means such as jacketing, cabling or bundling the fiber to cause random distortions of the waveguide axis.
A second dispersion reducing effect, which is discussed in the publication by D. Gloge et al., entitled "Multimode Theory of Graded-Core Fibers", published in the November 1973 issue of the Bell System Technical Journal, pp. 1563-1578, employs a graded, continuous index profile that varies from a maximum value on-axis to a lower value at the fiber surface. The index distribution in this type of waveguide is given by the equations EQU n(r) = n.sub.1 [1-2 .DELTA.(r/a).sup..alpha. ].sup.1/2 for r&lt;a (1) EQU n(r) .perspectiveto. n.sub.1 [1-2.DELTA.].sup.1/2 = n.sub.2 for r&gt;a (2)
where n.sub.1 is the on-axis refractive index, n.sub.2 is the cladding refractive index, .DELTA. = (n.sub.1 -n.sub.2)/n.sub.1, a is the core radius and .alpha. is a parameter between 1 and .infin.. As a result of mode coupling or the use of graded index profiles or some combination thereof, dispersion resulting from group velocity differences can be reduced sufficiently that the wavelength dependent effects, viz. material dispersion and dispersion within each mode due to the wavelength dependence of the modal group velocity, become the dominant source of dispersion if sources with broad spectral widths are used.